Because remaking the same features from scratch was taking too long. They had already delayed the project due to covid at that point. They ended up with three games: the one they started before intercept was created (and that never saw the light of day), the one based on KSP with the upgrades and new features added (also never seen publicly), a neutered version without the incomplete new features (like multeplayer and improved heat simulation) that was launched as early access. Poor fellows were set up for failure.